A cementitious dry cast mixture refers to pastes, mortars, and concrete compositions comprising a hydraulic cement binder having consistencies ranging from stiff to extremely dry as defined in ACI 211.3R-02, Table 2.1 (2007).
Pastes are defined as mixtures comprised of a hydraulic cement binder, either alone or in combination with pozzolans such as fly ash, silica fume, or blast furnace slag, and water. Mortars are defined as pastes that additionally include fine aggregate. Concretes additionally include coarse aggregate. These compositions may additionally include other admixtures such as set retarders, set accelerators, defoaming agents, air-entraining or air detraining agents, corrosion inhibitors, water reducing agents, pigments, and any other admixture to produce desired product performance properties.
Cementitious dry cast mixtures are used to form many articles, for example, concrete pipe, roof tile, masonry units, paver units, extruded plank, and any other preformed cementitious articles, in a mold or from an extrusion die. Each of these applications has basic desired characteristics that are critical in terms of producing quality finished units.
In masonry block applications, production speed, sufficient green strength, and the ability to resist slumping, sagging or deforming when stripped from the mold is important since stripping of the block from the mold occurs immediately after casting. The same is true for concrete pipe or roof tile with the additional desired property of improved surface appearance with reduced surface imperfections and reduced roller and/or die wear on equipment producing extruded pieces.
It is also desired to reduce the cycle time for the manufacture of a cementitious dry cast article. The reduction of cycle time reduces the cost of manufacture for each article and increases the number of articles that can be produced in a given period of time. Cycle time is defined as the time to complete one full cycle from the beginning of feed to the end, or next beginning of feed. The beginning of feed is when the dry cast mixture is fed from a collection hopper into the process. It is also desired to improve the compaction and consolidation of the cementitious dry cast mixture without altering the consistency of the mixture.
Green strength refers to the stability of the article in retaining its shape once the article is removed from the mold or extruder. Green strength depends, in part, on the consistency of the cementitious dry cast mixture, the amount of fines in the cementitious dry cast mixture, and the moldability of the cementitious dry cast mixture.
It is further desired to minimize the amount of water needed in a cementitious dry cast mixture to achieve consolidation and no sag or deformation in an article produced from the cementitious dry cast mixture.
A further limitation in the present art is the compressive strength of articles produced from cementitious dry cast mixtures. Early compressive strength is defined as the compressive strength achieved within 24 hours with or without steam cure. Compressive strength is determined by ASTM C-1176-2.
Known plasticizing admixtures for dry cast cementitious compositions often exhibit narrow ranges of performance where slight overdosing can create unworkable production mixtures, while slight underdosing can provide production problems in the manufacture of cementitious dry cast articles.
Additionally, many admixtures for dry cast cementitious compositions are formulated as oil-in-water emulsions and can be used as plasticizers in the production of dry cast cementitious articles. Oil-in-water emulsions, however, require either higher cost pre-emulsified starting raw materials or expensive production process. Furthermore, oil-in-water emulsions suffer from low product shelf-life, reduced environmental stability and increased susceptibility to microbial degradation.
During the production of dry cast articles, vibration energy is utilized to facilitate compaction and consolidation of the dry cast cementitious mixture. Due to the low workability of dry cast cementitious mixtures, however, generally high levels of vibration energy are required to properly and fully consolidate the mixture. When vibration is used, the paste fraction of the dry cast cementitious mixture liquefies providing a temporary increase in workability to the otherwise low workability mixture facilitating proper consolidation.
As the consolidation process continues, during the preparation of dry cast cementitious articles, large aggregate particles re-orient themselves such that more optimal packing occurs. The result of proper consolidation is a more uniform, void free material ensuring that the designed strength and durability of the dry cast article is achieved. Incomplete or improper consolidation can leave large voids or areas of honeycombing within the dry cast article leading to low compressive or flexural strengths, and poor surface finish.
What is needed in the industry is an additive for dry cast cementitious mixtures that operates over a wider effective dosage range, improves the mixture's response to vibration, and dampens out differences due to minor fluctuations from batch to batch. Improving the response to vibration would allow proper consolidation to be achieved with lower vibration energy, increased production or extrusion speed, and more consistent production of dry cast cementitious articles.